Sheet conveyance device

ABSTRACT

A sheet conveyance device that is capable of appropriately correcting a position of a sheet in the sheet width direction with respect to an image transferred thereon. A shift mechanism shifts a sheet conveyed in a sheet width direction perpendicular to a sheet conveyance direction. A first sensor detects one side edge of the sheet in the sheet width direction. A second sensor detects the other side edge of the sheet in the sheet width direction. A control unit drives the shift mechanism so that the center of the sheet in the sheet width direction is coincident with a predetermined target position based on the gap of the first and second sensors, detection timing of the one side edge by the first sensor, detection timing of the other side edged by the second sensor, and a predetermined standard value of sheet width.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet conveyance device, and particularly relates to a technique for correcting a position of a sheet with respect to an image that is formed by an image forming apparatus in a sheet width direction (a direction perpendicular to a sheet conveyance direction).

2. Description of the Related Art

Image forming apparatuses, such as copiers and printers, need a control (lateral registration) for adjusting a position of a conveyed sheet in the sheet width direction to a predetermined position for the purpose of an alignment with an image and proper arrangement of an ejected sheet bundle. With respect to such a demand, Japanese Laid-Open Patent Publication (Kokai) No. 2008-189395 (JP 2008-189395A) discloses a control method that detects one side edge of a sheet with a line sensor and shifts rollers that are grasping the sheet in the sheet width direction by a distance corresponding to a difference between the detected side edge and a target position, for example.

Since the accuracy required for a position correction of the conveyed sheet in the sheet width direction becomes higher in recent years, the conventional control method as disclosed in the publication is becoming difficult to satisfy such accuracy.

For example, the control method of the publication adjusts one side edge (a position of one of sides that are almost parallel to the sheet conveyance direction) of a sheet to a predetermined target position. Accordingly, when an actual sheet width deviates from a standard value due to reasons, such as a cutting error of sheet and contraction of sheet after fixing process, the center of an image transferred to a sheet deviates from the center of the sheet in the sheet width direction. When the sheet deviates from a suitable position for an image transferred, the appearance of printed matter deteriorates.

SUMMARY OF THE INVENTION

The present invention provides a sheet conveyance device that is capable of appropriately correcting a position of each sheet in the sheet width direction with respect to an image transferred thereon or with respect to other sheets that form a sheet bundle.

Accordingly, a first aspect of the present invention provides a sheet conveyance device comprising a shift mechanism configured to shift a sheet conveyed in a sheet width direction perpendicular to a sheet conveyance direction, a first sensor configured to detect one side edge of the sheet in the sheet width direction, a second sensor configured to be arranged at a different position from the first sensor in the sheet width direction and to detect the other side edge of the sheet in the sheet width direction, and a control unit configured to drive the shift mechanism so that the center of the sheet in the sheet width direction is coincident with a predetermined target position based on the gap of the first and second sensors, detection timing of the one side edge by the first sensor during a shift of the sheet in the sheet width direction, detection timing of the other side edged by the second sensor during the shift of the sheet, and a predetermined standard value of sheet width.

Accordingly, a second aspect of the present invention provides a sheet conveyance device comprising a shift mechanism configured to shift a sheet conveyed in a sheet width direction perpendicular to a sheet conveyance direction, a side-edge detection unit configured to detect a side edge of the sheet in the sheet width direction, a difference detection unit configured to detect difference between an actual sheet width of the sheet and a predetermined standard value in the sheet width direction, and a control unit configured to drive the shift mechanism so that the sheet is shifted to a predetermined target position in the sheet width direction based on the side edge of the sheet detected by the side-edge detection unit and the difference detected by the difference detection unit.

According to the present invention, deviation of the sheet width from the standard value and a delicate change about the sheet of which the position in the sheet width direction will be corrected are accurately detectable. When the position of the sheet in the sheet width direction is corrected based on the detected information, the position of each sheet in the sheet width direction with respect to an image formed by an image forming unit or with respect to other sheets that form a sheet bundle can be corrected appropriately.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a configuration of an image forming apparatus that is provided with a sheet conveyance device according to an embodiment of the present invention.

FIG. 2 is a view showing the detailed configuration of a lateral registration unit with which the sheet conveyance device of the embodiment is provided.

FIG. 3A, FIG. 3B, and FIG. 3C are views schematically showing a lateral registration process for an A4 sheet by the lateral registration unit in FIG. 2.

FIG. 4 is a view showing a positional relationship between a sheet of which a width deviates from a standard value of A4 size and a standard sensor as compared with the state in FIG. 3B.

FIG. 5 is a view showing a state where the lateral registration has been applied to the sheet of which a width deviates from a standard value of A4 size as compared with the state in FIG. 3C.

FIG. 6A, FIG. 6B, and FIG. 6C are views schematically showing a lateral registration process for a B5 sheet by the lateral registration unit in FIG. 2.

FIG. 7 is a view showing a positional relationship between a sheet of which a width deviates from a standard value of B5 size and the standard sensor as compared with the state in FIG. 6B.

FIG. 8 is a view showing a state where the lateral registration has been applied to the sheet of which a width deviates from a standard value of B5 size as compared with the state in FIG. 6C.

FIG. 9 is a timing chart showing a pulse output control process in the lateral registration control unit in FIG. 2.

FIG. 10 is a flowchart showing a first section of a lateral registration control process executed by the lateral registration control unit in FIG. 2.

FIG. 11 is a flowchart showing a second section of the lateral registration control process executed by the lateral registration control unit in FIG. 2.

FIG. 12 is a flowchart showing a third section of the lateral registration control process executed by the lateral registration control unit in FIG. 2.

DESCRIPTION OF THE EMBODIMENTS

Hereafter, embodiments according to the present invention will be described with reference to the attached drawings. A sheet conveyance device according to the present invention corrects a position of a sheet in the sheet width direction (referred to as a “lateral registration”, hereafter) with respect to an image that is transferred onto the sheet in an image forming apparatus, for example. This embodiment describes an image forming apparatus that is provided with the sheet conveyance device according to the present invention.

FIG. 1 is a view schematically showing a configuration of the image forming apparatus that is provided with the sheet conveyance device according to the embodiment of the present invention. The image forming apparatus is provided with an image forming unit 300 that forms an image on a sheet 30, and a sheet feeding unit 301 that feeds the sheet 30 to the image forming unit 300.

The image forming unit 300 has a photosensitive drum 16, a laser scanner 4, an electrostatic charger 20, a development device 22, and a cleaner 26. The photosensitive drum 16 as an image bearing member is driven so as to rotate in the direction of arrow B (CCW) shown in FIG. 1 by a motor (not shown). The laser scanner 4 irradiates the photosensitive drum 16 with a laser beam based on image information, and forms an electrostatic latent image on the photosensitive drum 16.

The electrostatic charger 20 electrifies the photosensitive drum 16 uniformly. The electrostatic charger 20 is mounted at the upstream of the laser beam irradiation position by the laser scanner 4 in the rotation direction of the photosensitive drum 16. The development device 22 develops the electrostatic latent image formed on the photosensitive drum 16 with toner, and forms a toner image. The development device 22 is arranged at the downstream of the laser beam irradiation position in the rotation direction of the photosensitive drum 16. The cleaner 26 collects the toner that remained on the photosensitive drum 16 without being transferred to a transfer belt 14 from the photosensitive drum 16 by a primary transferring charging unit 24 mentioned below.

The image forming unit 300 has the transfer belt 14, the primary transferring charging unit 24, and a secondary transfer roller 28. The endless transfer belt 14 is wound around a roller 12 and is driven by the roller 12 so as to rotate in the direction of arrow A shown in FIG. 1. The primary transferring charging unit 24 is arranged at a position opposite to the photosensitive drum 16 across the transfer belt 14. The primary transferring charging unit 24 transfers a toner image 31 formed on the photosensitive drum 16 to the transfer belt 14. The secondary transfer roller 28 transfers the toner image 31 transferred by the transfer belt 14 onto the sheet 30.

The sheet feeding unit 301 has a cassette 50 and a sheet conveyance device 302. The cassette 50 is removably attached to a main body (not shown) of the image forming apparatus, and accommodates the sheet 30. The sheet 30 is a recording sheet, an OHP sheet, or the like, specifically. The sheet 30 is fed from the cassette 50 towards the image forming unit 300 by a feed roller 51. The size of the sheet accommodated in the cassette 50 is beforehand set up by a user from an operation unit 1000, and is memorized in a memory in a controller 8. The size set up is a regular size, such as A3, A4, or B5, which is a standard value.

The sheet conveyance device 302 is mounted between the sheet feeding unit 301 and the image forming unit 300. The sheet 30 picked out from the cassette 50 by the feed roller 51 is fed to a skew correction unit 303 by conveying rollers 52 and 53.

The skew correction unit 303 that corrects skew of the sheet 30 has a skew correction control unit 9, a pair of skew correction rollers 2, and a pair of front end detection sensors 6. In this embodiment, the front end detection sensors 6 are aligned in the width direction of the sheet 30 (the direction perpendicular to a sheet conveyance direction and parallel to the sheet) at a predetermined interval, and detect the skew of the front side of the sheet with respect to the sheet width direction.

The skew correction control unit 9 calculates the skew of the sheet 30 based on the signals from the front end detection sensors 6, and outputs control pulses for the skew correction rollers 2 to a driver (not shown) of the skew correction rollers 2. The skew correction rollers 2 are aligned in the sheet width direction at a predetermined interval. The driver controls rotational speeds of the skew correction rollers 2 independently according to the received control pulses. Thereby, the skew of the sheet 30 is corrected.

The sheet 30 is conveyed from a skew correction unit 303 to a lateral registration unit 304. The lateral registration unit 304 corrects a lateral deviation of the sheet 30. The lateral registration unit 304 has a lateral-registration control roller (referred to as a “control roller”), a pair of lateral-registration standard sensors (referred to as “standard sensors”) 5, a lateral-registration timing sensor (referred to as a “timing sensor”) 10, and a lateral registration control unit 3.

The position in the sheet width direction (referred to as a “lateral position”) of the sheet is changed by shifting the control roller 1 that pinches the sheet 30 in the sheet width direction. The timing sensor 10 is a front end sensor that detects the front end of the sheet 30 conveyed. The lateral registration control unit 3 determines a timing at which the lateral registration starts based on the detection signal from the timing sensor 10 that detects the sheet 30. The lateral registration control unit 3 calculates a shift distance of the control roller 1 for correcting the lateral deviation of the sheet 30 based on the detection signals from the standard sensor 5 that detect the sheet 30, and controls the shift action of the control roller 1. Details will be described later.

The lateral registration control unit 3 outputs drive pulses for controlling the shift action of the control roller 1 to a driver (not shown) of the control roller 1 based on the signals from the standard sensors 5 and the timing sensor 10. The driver drives a motor (not shown) that moves a shaft of the control roller 1 in the width direction according to the received drive pulses. Thereby, the lateral deviation of the sheet 30 is corrected.

The skew correction and the lateral registration mentioned above increase accuracy in orientation and position of the sheet 30 with respect to the toner image 31 on the transfer belt 14. It should be noted that details of the lateral registration of the sheet 30 in the lateral registration unit 304 will be described later.

The sheet 30 is conveyed from the lateral registration unit 304 towards a nip position between the secondary transfer roller 28 and the transfer belts 14, and the toner image 31 on the transfer belt 14 is transferred onto the sheet at the nip position.

An image control unit 7 receives a laser beam detection signal from the laser scanner 4, and transmits image pulses corresponding to image data to the laser scanner 4 in synchronization with the received laser beam detection signal. The laser beam detection signal is generated when a laser beam reflected by a polygon mirror that deflects the laser beam built in the laser scanner 4 is detected by a laser beam detection sensor (not shown). The controller 8 temporarily stores image data transmitted from a PC or a reader (not shown), and transmits the image data to the image control unit 7 in response to an image request signal and a horizontal synchronizing signal from the image control unit 7.

It should be noted that the image control unit 7 generates the horizontal synchronizing signal based on the laser beam detection signal. When receiving a trigger signal from a CPU (not shown) that manages sequences of the entire apparatus, the image control unit 7 generates the image request signal, for example. After counting a predetermined number of horizontal synchronizing signals on the basis of the image request signal, the controller 8 transmits the image data to the image control unit 7 for every predetermined line in synchronization with the horizontal synchronizing signal. The image data is converted into the image pulses of which pulse width corresponds to data level in the image control unit 7.

When receiving the trigger signal indicating that an image will be formed from the CPU (not shown), the image control unit 7 outputs the image request signal to the controller 8. The controller 8 transmits the image data to the image control unit 7 based on the image request signal in synchronization with the horizontal synchronizing signal. The image control unit 7 transmits the image pulses corresponding to the received image data to the laser scanner 4.

The laser scanner 4 irradiates the photosensitive drum 16, which rotates in the direction shown by the arrow B in FIG. 1, with the laser beam corresponding to the received image pulses or the laser beam modulated based on the image data corresponding to the data from an image memory (not shown). At this time, the photosensitive drum 16 is beforehand charged uniformly by the electrostatic charger 20. Accordingly, an electrostatic latent image is formed on the photosensitive drum 16 when it is irradiated with the laser beam.

The electrostatic latent image is developed by the development device 22, and, thereby, a toner image is formed. The toner image formed on the photosensitive drum 16 is transferred onto the transfer belt 14 by a function of the primary-transfer-bias voltage impressed by the primary transferring charging unit 24.

On the other hand, the feed roller 51 feeds the sheet 30 from the cassette 50 to the image forming unit 300 in synchronization with the trigger signal generated by the CPU. The trigger signal is generated at timing when the position of the sheet 30 appropriately matches the toner image 31 while transferring to the sheet 30. The sheet 30 is conveyed to the conveying roller 53 through the conveying rollers 52 arranged at two places. Sensors (not shown) that detect the sheet 30 are arranged close to the conveying rollers 52, respectively. The CPU drives the conveying rollers 52 through a drive control unit (not shown) based on sheet-passage-detection signals from these sensors.

The sheet 30 is conveyed to the skew correction unit 303 by the conveying roller 53. The skew of the sheet 30 is corrected when passing the skew correction unit 303. In the skew correction unit 303, the skew correction control unit 9 calculates the time lag between the detection timing of the front end detection sensors 6 that detect the front end of the sheet 30 as a skew amount. The skew correction control unit 9 corrects a skew by decelerating one roller at the preceded side of the sheet among the skew correction rollers 2 that are aligned in the sheet width direction at the fixed interval. It should be noted that the skew correction control unit 9 calculates a slowdown target speed so that the integral value of the speed difference between the constant speed and the slowdown target speed in a deceleration period matches the skew amount.

The sheet 30 to which the skew correction has been applied in the skew correction unit 303 is conveyed to the lateral registration unit 304. The lateral registration to the sheet 30 is performed in the lateral registration unit 304. The configuration of the lateral registration unit 304 and lateral registration control are described in detail later.

The sheet 30 that passed through the lateral registration unit 304 is caught between the transfer belt 14 and the secondary transfer roller 28 and is conveyed, and the toner image 31 on the transfer belt 14 is transferred onto the sheet 30 by the secondary transfer roller 28 during the conveyance. Then, the sheet 30 is conveyed to a fixing unit (not shown). The fixing unit fixes the toner image 31 to the sheet 30 by heating and pressurizing.

FIG. 2 is a view showing the detailed configuration of the lateral registration unit 304. A section surrounded with a broken line in FIG. 2 is the lateral registration control unit 3 shown in FIG. 1. A timing chart showing a pulse output control process in the lateral registration control unit 3 is shown in FIG. 9. In the following description related with FIG. 2, FIG. 9 is referred to suitably.

The pair of standard sensors 5 shown in FIG. 1 are shown as a first standard sensor 5A and a second standard sensor 5B in FIG. 2. In this embodiment, the arrangement interval between the standard sensors 5A and 5B is 275 mm. The interval of 275 mm is an almost middle value between the width 297 mm of the A4 size and the width 257 mm of the B5 size.

In the skew correction unit 303, the front end of the sheet 30 of which the skew was corrected is detected by the timing sensor 10 while being conveyed by the control roller 1. Each of the standard sensors 5A and 5B and the timing sensor 10 outputs a signal of “1” in an ON state when detecting the sheet 30, and outputs a signal of “0” in an OFF state when not detecting the sheet 30.

In a preliminary shift operation and a lateral registration operation mentioned later, the standard sensor 5A switches to the ON state when detecting one side edge (close to the standard sensor 5A) of the sheet 30. In the similar manner, the standard sensor 5B switches to the ON state detecting the other side edge (close to the standard sensor 5B) of the sheet 30.

In the lateral registration control unit 3, when receiving the signal of “1” from the timing sensor 10, a trigger generation unit 100 determines the respective states of the standard sensors 5A and 5B. When the sheet 30 is in a state (1) in FIG. 2, the standard sensor 5A is in the OFF state, and the standard sensor 5B is in the ON state. At this time, the signal of “0” is inputted into the trigger generation unit 100 via an inverter 200 and an AND gate 201.

When determining that the signal from the AND gate 201 is “0” when the signal of “1” is received from the timing sensor 10, the trigger generation unit 100 transmits a preliminary shift start signal to a pulse generation unit 101 (time t1 in FIG. 9). The preliminary shift start signal makes the control roller 1 perform the preliminary shift operation for shifting the sheet 30 in the state (1) in FIG. 2 in a direction of an arrow R. The preliminary shift operation shifts the control roller 1 that pinches the sheet 30 in the direction of the arrow R so that the standard sensor 5A is ON and the standard sensor 5B is OFF as shown in the state (2) in FIG. 2.

When receiving the preliminary shift start signal, the pulse generation unit 101 outputs drive pulses and a CW (normal rotation)/CCW (reverse rotation) signal of a low level (=“0”) to a driver 40 for driving a motor 41 that shifts the control roller 1. At this time, the pulse generation unit 101 generates the drive pulses for the motor 41 so that the pulse rate per second varies from “P0” to “P1” (time t1 through time t2 in FIG. 9). The CW/CCW signal controls the direction of rotation of the motor 41.

When receiving the pulses, the driver 40 rotates the motor 41 in the normal direction when the CW/CCW signal is “0”, and rotates the motor 41 in the reverse direction when the CW/CCW signal is “1”. The shaft of the control roller 1 is connected with a belt 42 by a connecting member 44, and the belt 42 is looped over the shaft of the motor 41 and a shaft 43. When the motor 41 rotates, the belt 42 moves, which slides the shaft of the control roller 1 in its longitudinal direction. That is, the rotation of the shift of the motor 41 is converted into the linear movement of the control roller 1, which achieves the shift operation of the control roller 1 in the sheet width direction. In the preliminary shift operation, the motor 41 rotates in the normal direction, and the control roller 1 shifts in the direction of the arrow R.

The preliminary shift operation continues until the standard sensor 5A becomes the ON state and the standard sensor 5B becomes the OFF state. When the standard sensor 5A becomes the ON state and the standard sensor 5B becomes the OFF state, the output from the AND gate 201 becomes “1”. Receiving the output of “1” from the AND gate 201, the trigger generation unit 100 transmits a preliminary shift stop signal to the pulse generation unit 101 (time t3 in FIG. 9).

When the preliminary shift stop signal is received, the pulse generation unit 101 generates the drive pulses for the motor 41 so that the pulse rate per second varies from “P1” to “P0” (time t3 through time t4 in FIG. 9). In this way, the preliminary shift operation is finished. As a result of the operation of the preliminary shift operation, the sheet 30 moves to the state (2) shown in FIG. 2. That is, the side edge of the sheet 30 at the side of the standard sensor 5A certainly moves in the direction of the arrow R beyond the standard sensor 5A.

After finishing the preliminary shift operation, the trigger generation unit 100 transmits a lateral registration shift start signal that directs starting a lateral registration operation to the pulse generation unit 101. Receiving the lateral registration shift start signal, the pulse generation unit 101 outputs pulses and the CW/CCW signal of high level (=“1”) (time t5 in FIG. 9).

At this time, the pulse generation unit 101 generates the drive pulses for the motor 41 so that the pulse rate per second varies from “P0” to “P2” (time t5 through time t6 in FIG. 9). Thereby, the drive transmission mechanism that relates to the shift drive for the control roller 1 operates, and the sheet 30 shifts in the direction of the arrow L together with the control roller 1.

In this embodiment, since the pulse rate is changed to “P2” before the standard sensor 5A becomes the OFF state because the side edge of the sheet 30 at the side of the standard sensor 5A passes over the standard sensor 5A, the shift speed of the sheet 30 reaches a constant speed.

Since the lateral registration of the sheet 30 after the time t6 varies dependent on the relationship between the width of the sheet 30 and the gap of the standard sensors 5A and 5B, some cases will be separately described. In any case, it is controlled so that the center of the toner image 31 on the transfer belt 14 is coincident with the center of the sheet 30 in the width direction. In this embodiment, a lateral registration target position is set at a position away from the standard sensor 5A by 180 mm in the sheet width direction. Then, the shift control for the control roller 1 is performed so that the center of the sheet 30 in the width direction is coincident with the lateral registration target position.

The case where the sheet 30 is A4 size will be described with reference to FIG. 3A, FIG. 3B, and FIG. 3C. First, the case where the size of the sheet 30 is in agreement with the standard value of A4 size will be described.

FIG. 3A, FIG. 3B, and FIG. 3C are views schematically showing a lateral registration process for the sheet 30 of A4 size. The standard value of width of an A4 sheet is 297 mm, which is longer than the gap of 275 mm of the standard sensors 5A and 5B. Accordingly, as shown in FIG. 3A, when the standard sensor 5B detects the side edge of the sheet 30 during the shift of the sheet 30 in the direction of the arrow L, the standard sensor 5A keeps on detecting the sheet 30.

When both the outputs from the standard sensors 5A and 5B become “1”, the output from the AND gate 202 becomes “1”. On the other hand, as shown in FIG. 2, the lateral registration shift start signal outputted from the trigger generation unit 100 is inputted into an ON/ON counter 102. The ON/ON counter 102 is in a state that permits counting, when the lateral registration shift start signal is inputted. The ON/ON counter 102 starts counting the pulses when receiving the output of “1” from the AND gate 202 as an enable signal.

Then, when the sheet 30 shifts to the state in FIG. 3B, the standard sensor 5A varies from the ON state to the OFF state at the timing of detecting the sheet edge and the output from the standard sensor 5A becomes “0”. As a result of this, the output from the AND gate 202 becomes “0”. The ON/ON counter 102 stops counting the pulses in response to the output “0” from the AND gate 202.

When the width of the sheet 30 is equal to 297 mm that is the standard value of the A4 size, the distance from the standard sensor 5B to the side edge of the sheet 30 at the side of the standard sensor 5B is 22 mm as shown in FIG. 3B. Accordingly, the counted value of the ON/ON counter 102 is equal to the value that is obtained by dividing 22 mm by the shift distance of the sheet 30 per one pulse. For example, when the shift distance of the sheet 30 per one pulse is equal to 0.05 mm, the counted value will become “440 (=22/0.05)”. Thus, in this embodiment, the shift distance of the control roller 1 (the shift distance of the sheet 30) in the lateral registration operation is determined based on the shift distance of the control roller 1 per one pulse and the counted value.

The ON/ON counter 102 outputs the counted value, which shows the detected distance at which the standard sensors 5A and 5B become ON during the lateral registration operation, to a shift-pulse-number adjustment unit 105 as an ON/ON pulse number. The shift-pulse-number adjustment unit 105 outputs an adjusted lateral registration shift pulse number as follows to the pulse generation unit 101 in response to the ON/ON pulse number from the ON/ON counter 102.

That is, the controller 8 delivers an ON/ON standard pulse number and an OFF/OFF standard pulse number to the shift-pulse-number adjustment unit 105 corresponding to the size of the sheet conveyed.

The ON/ON standard pulse number is the number of pulses that are outputted while both the standard sensors 5A and 5B are in the ON state during the lateral registration operation, when a sheet that is coincident with a standard value of a certain size is conveyed. Since the shift distance of the sheet per pulse is constant, the ON/ON standard pulse number shows the shift distance of the sheet that is shifted while the standard sensors 5A and 5B keep the ON state during the lateral registration operation.

Similarly, the OFF/OFF standard pulse number is the number of pulses that are outputted while both the standard sensors 5A and 5B are in the OFF state during the lateral registration operation, when a sheet that is coincident with a standard value of a certain size is conveyed. Since the shift distance of the sheet per pulse is constant, the OFF/OFF standard pulse number shows the shift distance of the sheet that is shifted while the standard sensors 5A and 5B keep the OFF state during the lateral registration operation.

Therefore, when an A4 sheet is conveyed, the ON/ON standard pulse number “440” and the OFF/OFF standard pulse number “0” are delivered to the shift-pulse-number adjustment unit 105.

On the other hand, the controller 8 delivers the lateral registration shift pulse number to the shift-pulse-number adjustment unit 105. The lateral registration shift pulse number is a number of pulses that is outputted from the standard sensor 5A when shifting the sheet coincident with the standard value, and shows a standard shift distance corresponding to the sheet size.

When receiving the ON/ON pulse number from the ON/ON counter 102 under such a condition, the shift-pulse-number adjustment unit 105 compares it with the ON/ON standard pulse number (i.e., compares a detected distance with a standard detected distance). When these pulse numbers are coincident with each other, the shift-pulse-number adjustment unit 105 determines that the width of the sheet under the lateral registration operation is coincident with the standard value. In this case, since the ON/ON pulse number “440” is equal to the ON/ON standard pulse number “440”, it is determined that the width of the sheet 30 is coincident with the standard value of A4 size.

When the width of the sheet 30 is coincident with the standard value of A4 size, the center of the toner image is located at a position away from the standard sensor 5A by 180 mm in the width direction as shown in FIG. 3C. Accordingly, in order to match the center of the sheet 30 of A4 size to the center of the toner image, the side edge of the sheet 30 (referred to as a “sheet edge”) at the side of the standard sensor 5A should be coincident with the position away from the standard sensor 5A by 31.5 mm (the standard shift distance).

Accordingly, the lateral registration shift pulse number “630 (=31.5 mm/0.05 mm)” is set. When the width of the sheet 30 is coincident with the standard value of A4 size, the shift-pulse-number adjustment unit 105 does not adjust the lateral registration shift pulse number, and transmits the adjusted lateral registration shift pulse number “630” to the pulse generation unit 101.

When receiving the signal of “0”, which shows that the standard sensor 5A is in the OFF state, from an inverter 206 after starting the lateral registration for the sheet 30, the pulse generation unit 101 starts to count pulses outputted by itself. The pulse generation unit 101 latches the adjusted lateral registration shift pulse number at the timing when receiving the signal of “1” outputted from an AND gate 207 (time t7 in FIG. 9). It should be noted that the AND gate 207 outputs the signal of “1” at the time t7 in FIG. 9 at which the standard sensor 5A turns OFF (the standard sensor 5B keeps the ON state).

After that, the pulse generation unit 101 decreases the shift speed of the sheet 30 (the control roller 1) by lowering the pulse rate from “P2” as shown in FIG. 9. Then, the pulse generation unit 101 stops outputting the pulse when the counting pulse number is coincident with the adjusted lateral registration shift pulse number “630” (time t8 in FIG. 9). A hatched area S in the timing chart of the pulse rate in FIG. 9 is equivalent to the pulse number “630” that is outputted after the sheet edge at the side of the standard sensor 5A passes the standard sensor 5A. The product of the pulse number “630” and the distance per pulse (0.05 mm) is 31.5 mm. In this way, the sheet 30 reaches the position shown in FIG. 3C, and the lateral registration operation for the sheet 30 is finished.

Next, the case where the actual sheet width of the sheet 30 is not coincident with the standard value is described. Cutting error of the sheet 30 and contraction by heat fixing for a first surface when forming images on both surfaces become causes of a mismatch between the actual width of the sheet 30 and the standard value.

FIG. 4 is a view showing the positional relationship between the sheet 30 of A4 size of which the width is not coincident with the standard value and the standard sensor 5B as compared with the state in FIG. 3B. FIG. 5 is a view showing the state where the lateral registration has been applied to the sheet 30 of A4 size of which the width is not coincident with the standard value as compared with the state in FIG. 3C. It should be noted that FIG. 4 shows the vicinity of the standard sensor 5B only and FIG. 5 shows the vicinity of the standard sensor 5A only.

In the state shown in FIG. 4, the sheet edge at the side of the standard sensor 5A is coincident with the standard sensor 5A (see FIG. 3B). When the width of the sheet is shorter than the standard value by ΔD1 mm, the ON/ON pulse number outputted by the ON/ON counter 102 becomes less than “440” by “ΔD1/0.05”. The ON/ON pulse number “440−ΔD1/0.05” is transmitted to the shift-pulse-number adjustment unit 105.

The shift-pulse-number adjustment unit 105 compares the ON/ON pulse number “440−ΔD1/0.05” with the ON/ON standard pulse number “440”, and as a result, determines that the width of the sheet 30 is shorter than the standard value. In this case, since the required shift distance is (31.5+ΔD1/2) mm, the shift-pulse-number adjustment unit 105 transmits the pulse number that is larger than “630” by “(ΔD1/2)/0.05” to the pulse generation unit 101 as an adjusted lateral registration shift pulse number.

Thereby, the sheet edge at the side of the standard sensor 5A is adjusted to the position away from the standard sensor 5A by (31.5+ΔD1/2) mm as shown in FIG. 5. Thus, the center of the toner image is coincident with the center of the sheet 30 in the width direction at the position away from the standard sensor 5A by 180 mm as with the case where the width of the sheet 30 matches the standard value.

On the other hand, when the width of the sheet 30 is longer than the standard value by ΔD2 mm as shown in FIG. 4, the ON/ON pulse number outputted by the ON/ON counter 102 becomes more than “440” by “ΔD2/0.05”. The pulse number “440+ΔD2/0.05” is transmitted to the shift-pulse-number adjustment unit 105. The shift-pulse-number adjustment unit 105 compares the ON/ON pulse number “440+ΔD2/0.05” with the ON/ON standard pulse number “440”.

The shift-pulse-number adjustment unit 105 determines that the width of the sheet 30 is longer than the standard value, and transmits the pulse number that is less than “630” by “(ΔD2/2)/0.05” as the adjusted lateral registration shift pulse number to the pulse generation unit 101. Thereby, the sheet edge at the side of the standard sensor 5A is adjusted to the position away from the standard sensor 5A by (31.5−ΔD2/2) mm as shown in FIG. 5. Thus, the center of the toner image is coincident with the center of the sheet 30 in the width direction at the position away from the standard sensor 5A by 180 mm as with the case where the width of the sheet 30 matches the standard value.

The case where the sheet 30 is B5 size will be described with reference to FIG. 6A, FIG. 6B, and FIG. 6C. FIG. 6A, FIG. 6B, and FIG. 6C are views schematically showing a lateral registration process for the sheet 30 of B5 size. The width of the sheet of B5 size is 257 mm, which is shorter than the gap of 275 mm of the standard sensors 5A and 5B. As shown in FIG. 6A, at the moment when the standard sensor 5A turns OFF during the lateral registration operation for the sheet 30, the standard sensor 5B still keeps the OFF state.

The lateral registration shift start signal outputted from the trigger generation unit 100 is inputted also into an OFF/OFF counter 103 (see FIG. 2). When inputting the lateral registration shift start signal, the OFF/OFF counter 103 is permitted to count.

When both the outputs from the standard sensors 5A and 5B are “0”, the output from an AND gate 205 through inverters 203 and 204 is “1”. The ON/ON counter 103 starts counting the pulses when receiving the output of “1” from the AND gate 205 as an enable signal.

Then, when the sheet 30 is further shifted and the standard sensor 5B turns ON as shown in FIG. 6B, the output from the AND gate 205 becomes “0”. The OFF/OFF counter 103 stops counting the pulses in response to the output “0” from the AND gate 205.

When the width of the sheet 30 is equal to 257 mm that is the standard value of the B5 size, the distance from the standard sensor 5A to the sheet edge at the side of the standard sensor 5A is 18 mm as shown in FIG. 6B. Accordingly, the counted value of the OFF/OFF counter 103 is equal to the value that is obtained by dividing 18 mm by the shift distance of the sheet 30 per one pulse. Since the shift distance of the sheet 30 per one pulse is 0.05 mm as mentioned above, the count value that the OFF/OFF counter 103 outputs is “360 (=18/0.05)”.

The OFF/OFF counter 103 outputs the counted value, which shows the detected distance in which the standard sensors 5A and 5B keep the OFF state during the lateral registration operation, to the shift-pulse-number adjustment unit 105 as an OFF/OFF pulse number. The shift-pulse-number adjustment unit 105 outputs an adjusted lateral registration shift pulse number as follows to the pulse generation unit 101 in response to the OFF/OFF pulse number from the OFF/OFF counter 103.

When a B5 sheet is conveyed, a CPU (not shown) delivers the OFF/OFF standard pulse number “360” and the ON/ON standard pulse number “0” to the shift-pulse-number adjustment unit 105. When receiving the OFF/OFF pulse number from the OFF/OFF counter 103, the shift-pulse-number adjustment unit 105 compares it with the OFF/OFF standard pulse number. When these pulse numbers are coincident with each other, the shift-pulse-number adjustment unit 105 determines that the width of the sheet under the lateral registration operation is coincident with the standard value. In this case, since the OFF/OFF pulse number “360” is equal to the OFF/OFF standard pulse number “360”, it is determined that the of the sheet 30 is coincident with the standard value of B5 size.

As shown in FIG. 6C, the center of the toner image is located at the position away from the standard sensor 5A by 180 mm in the width direction. Accordingly, in order to match the center of the sheet 30 of B5 size to the center of the toner image, the sheet edge at the side of the standard sensor 5A should be coincident with the position away from the standard sensor 5A by 51.5 mm.

Accordingly, the lateral registration shift pulse number “1030 (=51.5/0.05)” is set. When the width of the sheet 30 of B5 size is coincident with the standard value of B5 size, the shift-pulse-number adjustment unit 105 does not adjust the lateral registration shift pulse number, and transmits the adjusted lateral registration shift pulse number “1030” to the pulse generation unit 101.

When receiving the signal of “0”, which shows that the standard sensor 5A is in the OFF state, from the inverter 206 after starting the lateral registration operation for the sheet 30, the pulse generation unit 101 starts to count pulses outputted by itself. The pulse generation unit 101 latches the adjusted lateral registration shift pulse number at the timing when receiving the signal of “1” outputted from the AND gate 207 (time t7 in FIG. 9).

After that, the pulse generation unit 101 decreases the pulse rate from “P2” as shown in FIG. 9, and stops outputting a pulse when the pulse number under counting reaches the adjusted lateral registration shift pulse number “1030” (time t8 in FIG. 9). The hatched area S in the timing chart of the pulse rate in FIG. 9 is equivalent to the pulse number “1030” that is outputted after the sheet edge at the side of the standard sensor 5A passes the standard sensor 5A. The product of the pulse number “1030” and the distance per pulse is 51.5 mm. In this way, the sheet 30 reaches the position shown in FIG. 6C, and the lateral registration operation for the sheet 30 is finished.

FIG. 7 is a view showing the positional relationship between the sheet 30 of B5 size of which the width is not coincident with the standard value and the standard sensor 5B as compared with the state in FIG. 6B. FIG. 8 is a view showing the state where the lateral registration has been applied to the sheet 30 of B5 size of which the width is not coincident with the standard value as compared with the state in FIG. 6C. It should be noted that FIG. 7 and FIG. 8 show the vicinity of the standard sensor 5A only.

In the state shown in FIG. 4, the sheet edge at the side of the standard sensor 5B is coincident with the standard sensor 5B (see FIG. 6B). When the width of the sheet 30 is longer than the standard value by ΔD3 mm, the OFF/OFF pulse number outputted by the OFF/OFF counter 103 becomes less than “360” by “ΔD3/0.0”. The OFF/OFF pulse number “360−ΔD3/0.05” is transmitted to the shift-pulse-number adjustment unit 105. The shift-pulse-number adjustment unit 105 compares the OFF/OFF pulse number “360−ΔD3/0.05” with the OFF/OFF standard pulse number “360”.

The shift-pulse-number adjustment unit 105 determines that the width of the sheet 30 is longer than the standard value, and transmits the pulse number that is less than “1030” by “(ΔD3/2)/0.05” as the adjusted lateral registration shift pulse number to the pulse generation unit 101. Thereby, the sheet edge at the side of the standard sensor 5A is adjusted to the position away from the standard sensor 5A by (51.5−ΔD3/2) mm as shown in FIG. 8. Thus, the center of the toner image is coincident with the center of the sheet 30 in the width direction at the position away from the standard sensor 5A by 180 mm as with the case where the width of the sheet 30 matches the standard value.

On the other hand, when the width of the sheet 30 is shorter than the standard value by ΔD4 mm as shown in FIG. 7, the OFF/OFF number outputted by the OFF/OFF counter 103 becomes more than “360” by “ΔD4/0.05”. The OFF/OFF pulse number “360+ΔD4/0.05” is transmitted to the shift-pulse-number adjustment unit 105. The shift-pulse-number adjustment unit 105 compares the OFF/OFF pulse number “360+ΔD4/0.05” with the ON/ON standard pulse number “440”.

The shift-pulse-number adjustment unit 105 determines that the width of the sheet 30 is shorter than the standard value, and transmits the pulse number that is more than “1030” by “(ΔD4/2)/0.05” as the adjusted lateral registration shift pulse number to the pulse generation unit 101. Thereby, the sheet edge at the side of the standard sensor 5A is adjusted to the position away from the standard sensor 5A by (51.5+ΔD4/2) mm as shown in FIG. 8. Thus, the center of the toner image is coincident with the center of the sheet 30 in the width direction at the position away from the standard sensor 5A by 180 mm as with the case where the width of the sheet 30 matches the standard value.

The above-mentioned lateral registration control executed by the lateral registration control unit 3 will be described with reference to flowcharts. FIG. 10, FIG. 11, and FIG. 12 are flowcharts of the lateral registration control executed by the lateral registration control unit 3. It should be noted that the lateral registration control is generalized and described and the width of a sheet is dealt with as a variable.

As shown in FIG. 10, the trigger generation unit 100 determines whether the timing sensor 10 detects the sheet that is corrected in skew by the skew correction unit 303 and is conveyed from the skew correction unit 303 (step S1). When the result is “NO” in the step S1, the trigger generation unit 100 repeats the step S1. When the result is “YES” in the step S1, the trigger generation unit 100 determines whether the standard sensor 5A is ON and the standard sensor 5B is OFF (step S2).

When the result is “NO” in the step S2, the trigger generation unit 100 transmits the preliminary shift start signal to the pulse generation unit 101. The pulse generation unit 101 controls a preliminary shift operation in response to the preliminary shift start signal (step S3). When the result is “YES” in the step S2, the trigger generation unit 100 transmits the lateral registration shift start signal to the pulse generation unit 101. The pulse generation unit 101 starts to control the lateral registration shift operation in response to the lateral registration shift start signal (step S4).

The lateral registration control unit 3 determines whether the standard sensor 5B turned ON (step S5). The following process executed when the result is “NO” in the step S5 will be described below with reference to FIG. 12. When the result is “YES” in the step S5, a pulse output is controlled as follows depending on the timing relationship between ON/OFF timings of the standard sensors 5A and 5B.

The lateral registration control unit 3 determines whether the standard sensor 5A turned OFF at the same time when the standard sensor 5B turned ON (step S7). When the result is “YES” in the step S7, the pulse generation unit 101 starts counting pulses (step S8). Then, the shift-pulse-number adjustment unit 105 checks whether the ON/ON standard pulse number is “0” (step S9).

When the standard value of width of the sheet conveyed is longer than the gap of the standard sensors 5A and 5B, the difference between the standard value of sheet width and the gap of the standard sensors 5A and 5B is defined as “δ”. A shift distance of the control roller 1 corresponding to the rotation of the shaft of the motor 41 when the pulse generation unit 101 outputs one pulse is defined as “γ”. Then, the ON/ON standard pulse number is denoted by “δ/γ”.

Accordingly, for example, assuming that the shift distance of the control roller 1 per pulse is 0.25 mm and the ON/ON standard pulse number is 10. In this case, the standard value of sheet width becomes longer than the gap of the standard sensors 5A and 5B by 2.5 mm. Accordingly, both the standard sensors 5A and 5B are in the ON state until ten pulses are outputted during the lateral-registration shift of this sheet.

In step S7, when it is determined that the standard sensor 5A turned OFF at the same time when the standard sensor 5B turns ON during the lateral-registration shift of sheet, the sheet width is equal to the gap of the standard sensors 5A and 5B. Then, when the result is “NO” in the step S9, i.e., when the ON/ON standard pulse number is not “0”, the sheet width is shorter than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·(ON/ON standard pulse number)”. Accordingly, the shift-pulse-number adjustment unit 105 transmits the pulse number, which is the sum of “lateral registration shift pulse number” and “(ON/ON standard pulse number)/2”, to the pulse generation unit 101 as the adjusted lateral registration shift pulse number (step S11).

The pulse generation unit 101 determines whether the received adjusted lateral registration shift pulse number is equal to the count value of the pulse generation unit 101 (step S12). When the result is “NO” in the step S12, the determination in the S12 is repeated. When the result is “YES” in the step S12, the lateral-registration shift is finished. Thus, the center of a toner image and the center of a sheet in a width direction agree.

When the result is “YES” in the step S9, i.e., when the ON/ON standard pulse number is “0”, the shift-pulse-number adjustment unit 105 checks whether the OFF/OFF standard pulse number is “0” (step S10). When the result is “NO” in the step S10, the standard value of sheet width is shorter than the gap of the standard sensors 5A and 5B by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·(OFF/OFF standard pulse number)”.

The sheet width is equal to the gap of the standard sensors 5A and 5B. Accordingly, the shift-pulse-number adjustment unit 105 determines that the sheet width is longer than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·(OFF/OFF standard pulse number)”. The shift-pulse-number adjustment unit 105 transmits the pulse number, which is obtained by subtracting “(OFF/OFF standard pulse number)/2” from the “lateral registration shift pulse number”, to the pulse generation unit 101 as the adjusted lateral registration shift pulse number (step S13).

When the result is “YES” in the step S10, i.e., when the OFF/OFF standard pulse number is “0”, the standard value of the sheet width is equal to the gap of the standard sensors 5A and 5B. In this case, the shift-pulse-number adjustment unit 105 determines that the sheet width is coincident with the standard value. Then, the shift-pulse-number adjustment unit 105 reflects the lateral registration shift pulse number to the control as the adjusted lateral registration shift pulse number without adjusting (step S14).

The process proceeds to the step S12 after the steps S13 and S14. The process in the step S12 has been already described, and the description thereof is omitted.

Next, the control process executed when the result is “NO” in the step S7 will be described with reference to FIG. 11 (the second flowchart). When the result is “NO” in the step S7 (when both the standard sensors 5A and 5B are in the ON state during the shift of the sheet), the ON/ON counter 102 starts counting pulses (step S15). Then, the ON/ON counter 102 determines whether the standard sensor 5A turned OFF (step S16).

When the result is “NO” in the step S16, the ON/ON counter 102 continues counting pulses until the standard sensor 5A turns OFF. When the result is “YES” in the step S16, the ON/ON counter 102 stops counting pulses, and transmits the count value to the shift-pulse-number adjustment unit 105 as the ON/ON pulse number. At the same time, the pulse generation unit 101 starts counting pulses (step S17).

Then, the shift-pulse-number adjustment unit 105 checks whether the ON/ON standard pulse number is “0” (step S18). When the result is “NO” in the step S18, the standard value of the sheet width is longer than the gap of the standard sensors 5A and 5B by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·(ON/ON standard pulse number)”. Accordingly, the shift-pulse-number adjustment unit 105 determines whether there the ON/ON pulse number is smaller than the ON/ON standard pulse number (step S19).

When the result is “YES” in the step S19, the sheet width is shorter than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·[(ON/ON standard pulse number)−(ON/ON pulse number)]”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is the sum of “the lateral registration shift pulse number” and “[(ON/ON standard pulse number)−(ON/ON pulse number)]/2”, as the adjusted lateral registration shift pulse number (step S20).

When the result is “YES” in the step S19, the sheet width is longer than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·[(ON/ON pulse number)−(ON/ON standard pulse number)]”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is obtained by subtracting “[(ON/ON pulse number)−(ON/ON standard pulse number)]/2” from “the lateral registration shift pulse number”, as the adjusted lateral registration shift pulse number (step S21).

The adjusted lateral registration shift pulse number determined in the step S20 or S21 is transmitted to the pulse generation unit 101, and then, the process proceeds to the step S12. The process in the step S12 has been already described, and the description thereof is omitted.

When the result is “YES” in the step S18, the shift-pulse-number adjustment unit 105 checks whether the OFF/OFF standard pulse number is “0” (step S32). When the result is “YES” in the step S32, the sheet width is longer than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·[(ON/ON pulse number)+(OFF/OFF standard pulse number)]”.

Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is obtained by subtracting “[(ON/ON pulse number)+(OFF/OFF standard pulse number)]/2” from “the lateral registration shift pulse number”, as the adjusted lateral registration shift pulse number (step S33).

When the result is “YES” in the step S32, both the ON/ON standard pulse number and the OFF/OFF standard pulse number are “0”. In this case, the sheet width is longer than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·(ON/ON pulse number)”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is obtained by subtracting “(ON/ON pulse number)/2” from “the lateral registration shift pulse number”, as the adjusted lateral registration shift pulse number (step S34).

The adjusted lateral registration shift pulse number determined in the step S33 or S34 is transmitted to the pulse generation unit 101, and then, the process proceeds to the step S12. The process in the step S12 has been already described, and the description thereof is omitted.

Next, the control process executed when the result is “NO” in the step S5 will be described with reference to FIG. 12 (the third flowchart). The lateral registration control unit 3 determines whether the standard sensor 5B turned OFF (step S6). When the result is “NO” in the step S6, the lateral registration control unit 3 returns the process to the step S5. When the result is “YES” in the step S6, the pulse generation unit 101 starts counting pulses, and the OFF/OFF counter 103 also starts counting pulses (step S22).

Then, the OFF/OFF counter 103 determines whether the standard sensor 5B turned ON (step S23). When the result is “NO” in the step S23, the trigger generation unit 103 repeats the step S23. When the result is “YES” in the step S23, the OFF/OFF counter 103 stops counting pulses, and transmits the count value to the shift-pulse-number adjustment unit 105 as the OFF/OFF pulse number (step S24).

The shift-pulse-number adjustment unit 105 checks whether the OFF/OFF standard pulse number is “0” (step S25). When the result is “NO” in the step S25, the shift-pulse-number adjustment unit 105 determines whether the OFF/OFF standard pulse number is larger than the OFF/OFF pulse number (step S26).

When the result is “YES” in the step S26, the sheet width is longer than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)˜[(OFF/OFF standard pulse number)−(OFF/OFF pulse number)]”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is obtained by subtracting “[(OFF/OFF standard pulse number)−(OFF/OFF pulse number)]/2” from “the lateral registration shift pulse number”, as the adjusted lateral registration shift pulse number (step S27).

When the result is “NO” in the step S26, the sheet width is shorter than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·[(OFF/OFF pulse number)−(OFF/OFF standard pulse number)]”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is the sum of “the lateral registration shift pulse number” and “[(OFF/OFF pulse number)−(OFF/OFF standard pulse number)]/2”, as the adjusted lateral registration shift pulse number (step S28).

The adjusted lateral registration shift pulse number determined in the step S27 or S28 is transmitted to the pulse generation unit 101, and then, the process proceeds to the step S12. The process in the step S12 has been already described, and the description thereof is omitted.

When the result is “YES” in the step S25, the shift-pulse-number adjustment unit 105 checks whether the ON/ON standard pulse number is “0” (step S29). When the result is “NO” in the step S29, the sheet width is shorter than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·[(ON/ON standard pulse number)+(OFF/OFF pulse number)]”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is the sum of “the lateral registration shift pulse number” and “[(ON/ON standard pulse number)+(OFF/OFF pulse number)]/2”, as the adjusted lateral registration shift pulse number (step S30).

When the result is “YES” in the step S29, the sheet width is shorter than the standard value by “(the shift distance of the control roller 1 when one pulse is given to the motor 41)·(OFF/OFF pulse number)”. Accordingly, the shift-pulse-number adjustment unit 105 determines the pulse number, which is the sum of “the lateral registration shift pulse number” and “(OFF/OFF pulse number)/2”, as the adjusted lateral registration shift pulse number (step S31).

The adjusted lateral registration shift pulse number determined in the step S30 or S31 is transmitted to the pulse generation unit 101, and then, the process proceeds to the step S12. The process in the step S12 has been already described, and the description thereof is omitted.

According to this embodiment, even when an actual sheet width deviates from a standard value due to reasons, such as a cutting error of sheet and contraction of sheet after fixing process, a position of the sheet in the sheet width direction with respect to an image formed with the image forming unit can be corrected to a proper position. It should be noted that a position of a sheet in the width direction with respect to an image formed with the image forming unit is also corrected to a proper position when the sheet width is coincident with the standard value.

Although the embodiments of the invention have been described, the present invention is not limited to the above-mentioned embodiments, the present invention includes various modifications as long as the concept of the invention is not deviated. The embodiments mentioned above show examples of the present invention, and it is possible to combine the embodiments suitably.

For example, although the standard sensors 5A and 5B are fixed in the above-mentioned embodiment, a distance variation mechanism may be provided so that the gap of the standard sensors 5A and 5B according to the standard value of the sheet conveyed. For example, when at least one of the standard sensors 5A and 5B is moved so that the gap of the standard sensors 5A and 5B is coincident with the standard width of the sheet conveyed, the algorithm for detecting the difference between the actual sheet width and the standard value can be simplified. The gap of the standard sensors 5A and 5B may be narrower or wider than the standard width of the sheet conveyed. That is, when the size relation between the gap of the standard sensors 5A and 5B and the standard width of the sheet conveyed is fixed, the algorithm for detecting the difference can be simplified regardless of a sheet size.

Although the side edge of the sheet is detected using one pair of standard sensors 5A and 5B in the above-mentioned embodiment, a plurality of pairs of standard sensors of which gaps are different may be arranged. In this case, the standard sensors to be used may be selected so that the size relation between the gap of the standard sensors and the standard width of the sheet conveyed does not vary.

In the above-mentioned embodiment, one pair of standard sensors 5A and 5B are used as the side edge detection unit that detects a side edge of a sheet and as the difference detection unit that detects difference between an actual sheet width and a standard value. The present invention is not limited to the above configuration. The side edge detection unit that detects a side edge of a sheet and the difference detection unit that detects difference between an actual sheet width and a standard value may be separately provided. In this case, the sheet conveyance device can shift a sheet to a predetermined target position in the sheet width direction based on the detection timing of the side edge of the sheet by the side edge detection unit and the difference detected by the difference detection unit.

Other Embodiments

Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium).

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2011-258771, filed on Nov. 28, 2011, which is hereby incorporated by reference herein in its entirety. 

What is claimed is:
 1. A sheet conveyance device comprising: a shift mechanism configured to shift a sheet conveyed in a sheet width direction perpendicular to a sheet conveyance direction; a first sensor configured to detect one side edge of the sheet in the sheet width direction; a second sensor configured to be arranged at a different position from said first sensor in the sheet width direction and to detect the other side edge of the sheet in the sheet width direction; and a control unit configured to drive said shift mechanism so that the center of the sheet in the sheet width direction is coincident with a predetermined target position based on the gap of said first and second sensors, detection timing of the one side edge by said first sensor during a shift of the sheet in the sheet width direction, and detection timing of the other side edged by said second sensor during the shift of the sheet.
 2. The sheet conveyance device according to claim 1, wherein said control unit determines difference between an actual sheet width of the sheet and a standard value of the sheet width based on the detection timing of the one side edge by said first sensor during the shift of the sheet in the sheet width direction, the detection timing of the other side edge by said second sensor during the shift of the sheet, and the standard value of the sheet width, determines a shift distance required for matching the center of the sheet in the sheet width direction with the target position based on the difference and a standard shift distance required for matching a center of a sheet of which width is equal to the standard value with the target position, and shifts the sheet by the shift distance.
 3. The sheet conveyance device according to claim 2, further comprising a front end sensor configured to detect the front end of the sheet, wherein said control unit performs a preliminary shift for shifting the sheet in the sheet width direction so that said second sensor does not detect the sheet and said first sensor detects the sheet, if said second sensor detects the sheet and said first sensor does not detect the sheet at the time when said front end sensor detects the front end of the sheet, and wherein said control unit shifts the sheet in a direction opposite to the shift direction of the preliminary shift so that the center of the sheet is coincident with the target position in the sheet width direction after finishing the preliminary shift.
 4. The sheet conveyance device according to claim 2, wherein said shift mechanism has a drive mechanism including a motor that shifts a conveyance roller, which conveys the sheet in the sheet conveyance direction, in the sheet width direction, and wherein said control unit determines the standard shift distance and the shift distance based on the number of pulses for driving the motor and a shift distance of the conveyance roller per one pulse.
 5. The sheet conveyance device according to claim 2, further comprising a distance variation mechanism configured to change the gap of said first and second sensors.
 6. The sheet conveyance device according to claim 2, wherein a plurality of pairs of the first and second sensors of which gaps in the sheet width direction are different are arranged.
 7. A sheet conveyance device comprising: a shift mechanism configured to shift a sheet in a sheet width direction perpendicular to a sheet conveyance direction; a side-edge detection unit configured to detect a side edge of the sheet in the sheet width direction; a difference detection unit configured to detect difference between an actual sheet width of the sheet and a standard value of the sheet width in the sheet width direction; and a control unit configured to drive said shift mechanism so that the sheet is shifted to a predetermined target position in the sheet width direction based on the side edge of the sheet detected by said side-edge detection unit and the difference detected by said difference detection unit.
 8. The sheet conveyance device according to claim 7, further comprising a setting unit configured to set up the size of the sheet, and wherein the standard value is determined based on the size set up by said setting unit.
 9. The sheet conveyance device according to claim 7, wherein said side-edge detection unit has a first sensor for detecting one side edge of the sheet and a second sensor for detecting the other side edge of the sheet, and the first and second sensors are aligned in the sheet width direction.
 10. The sheet conveyance device according to claim 9, wherein said control unit causes said shift mechanism to shift the sheet so that one of the first and second sensors does not detect the sheet and the other sensor detects the sheet, if the one of the sensors detects the sheet and the other sensor does not detect the sheet at the time when the sheet is conveyed to a predetermined position.
 11. The sheet conveyance device according to claim 9, wherein said difference detection unit detects the difference based on the gap of the first and second sensors and a shift distance of the sheet between a position where one of the first and second sensors detects the sheet and the other sensor does not detect the sheet and a position where the one of the sensors does not detect the sheet and the other sensor detects the sheet.
 12. The sheet conveyance device according to claim 7, wherein the predetermined target position is determined as a position at which the center of the sheet in the sheet width direction is coincident with a center of an image that is formed by an image forming unit and will be transferred onto the sheet. 